Method of rolling strip material



May 21, 1935.

METHOD OF ROLLING STRIP MATERIAL Filed Sept. 29, 1954' 2 Sheets-Sheet 12 2 5 4 J 5 7 a .9 J0 gg ifl 3 000017 0 01] gnaw gooodgoooog o7oo 0000?00003 oooog 0000000 I l 17 INVENTOR May 21, 1935. c. A. KRAL 2,002,266

METHOD OF ROLLING STRIP MATERIAL Filed Sept. 29, 1934 2 Sheets-Sheet 2THICKNESS //v INCHES PA ss 4 044 BER INVENTOR cm 8. KM

CONVEXI ry nv INCHES Patented May 21, 1935 UNITED STATES PATENT OFFICEMETHOD OF ROLLING STRIP MATERIAL Charles A. Kral, Steubenville, OhioApplication September 29, 1934, Serial No. 746,221

Claims.

This invention relates to a method of rolling strip material and isherein particularly described as applied to the rolling of wide, thinsteel in the formknown as broad strip or strip sheet.

5 Such material is widely used at the present time, both in the hotrolled and cold rolled condition. The hot rolled strip is produced in ahot mill from an ingot 6r a slab and the material thus formedconstitutes a valuable article for use either in its then state or as astarting material for cold rolling.

Generally speaking, this material is exceedingly wide in comparison withits thickness. It has been termed high ratio material and has beendistinguished from ordinary strip in that it has a width-to-thicknessratio in excess of 400 to 1. A great deal vof attention has been givento the problem of rolling high ratio strip, and especially to the rollcontours employed. It has been generally accepted as a fact that somedefinite relationship must exist between the contours of the successivepasses in the mill if the work piece is to track in a straight linetherethrough. According to the proponents of one system, the work pieceshould be originally convex and the. contour of the active passes shouldbe such that the amount of convexity decreases progressively from standto stand (see Tytus Reissue Patent No.- 16,884). According to theproponents of another system, the work piece should be originallyconcave and the active passes so arranged that the concavityprogressively decreases from stand to stand (see Corrigan and JenkinsPatent No. 1,829,339). According to still a third system, it is proposedthat the active passes in each stand shall be to all intents andpurposes flat so that the direction of travel of the piece may becontrolled by side guides (see Elliot Patent No. 1,754,745). A practical40 defect of all of these systems is that great care must be exercisedin originally determining and maintaining the requisite pass contours inthe several stands in order to carry out the intended system of rolling;The contour of the active pass depends, not only on the original contourof the .rolls, but also on a number of other factors. It has beenheretofore recognized that the following factors are involved:

(1) The prepared contour of the rolls.

'(2) The temperature of the rolls.

(3) The composition and springiness of the rolls. N

- (4) The spacing of the rolls, or'screw.

(5) The shape, composition and temperature of the work-piece.

In addition to the above,-I have determined that the factor of speed isan important one and should be taken into account if any system,dependent for its success upon the relative contour of successivepasses, is adopted. From the 5 foregoing, it will be seen that inpractical operation a great deal of experimenting and careful controlare essential to the successful operation of any system dependent forits success upon considerations of pass contour. The changing of rolls,in case they are not ground to the proper shape, necessitates ashut-down of the mill and a consequent loss of production. Experimentingwith roll settings and the like, as recommended by the proponents ofthese several systems, is also likely to prove wasteful of time, andhence expensive. Upon to the present time consideration has been givenonly to the active pass contours when the rolls are new, but in practicethe situation is further complicated by the factor of roll wear. All ofthe work rolls in the millwear during rolling, this being particularlymarked in hot rolling. The rolls in the several stands do not wear inthe same amount, nor is they original prepared contour of the rollsmaintained throughout their life.v In consequence, if the operator isrequired to maintain a determined relation between active pass contoursin the several stands, he must be constantly adjusting his mill to meetthe ever-changing situation brought about by roll wear. His diflicultieswill be fully appreciated when it is considered that certain of therolls in the mill may be worn out before others, and hence must bechanged oftener, so that the mill contains fresh rolls in some standsand worn rolls in another. These diificulties are to be expectedregardless of which of the three systems above referred to is adopted.

I have discovered that high ratio strip may be successfully rolled,either hot or cold, and that for practical purposes all considerationsof roll contour may be dispensed with, if a sufiicient number of passesis employed and the work is divided between the passes as hereinafterdescribed. A full understanding of my invention 45 requires a shortdescription of continuous rolling generally. In the continuous mill thepiece is passed successively from one stand to the next, being reducedfrom an initial work piece which,

in the case of bars or shapes, is a billet and, in 50 the case of strip,may be a slab or an ingot, and by successive passage through a series ofstands arranged in a continuous manner is reduced to the desired sizeand shape. It is necessary to provide a sufllcient number of stands toinsure that the grain refinement resulting from mechanical working isobtained, and it is also necessary to so arrange the reduction in theseveral stands that the piece travels smoothly from one stand to the.next. In any reducing stand the delivery speed is necessarily greaterthan the entering speed because the cross sectional area of the materialis reduced. It follows that the speed of the -metal progressivelyincreases as it travels through the mill, and the rolls must move atperipheral speeds which progressively increase from pass to pass inorder that the piece shall be properly rolled in each stand withoutpiling up or cobbling between passes.

From the foregoing it will be seen that the selection of a schedule ofreductions involves several factors but heretofore, so far as I amaware, it has never been known or understood that by properly selectingthe reductions in the several passes it is possible to eliminateconsiderations of pass contour and to roll high ratio material withoutadjustment for the many factors entering into all pass contour systems.By my invention it is possible to quickly and easily establish aschedule of reductions for the several stands in the mill and by sodoing to dispense with all the considerations which must ordinarily betaken into account in order to cause the material to track through themill.

The principle which I-have established is as followsz-The work pieceshould be reduced in such an amount in intermediate passes that thecurve in which thickness is plotted against pass number lies in anintermediate portion below an exponential curve of like characterconnecting the first and last points of the first-mentioned curve andrepresenting uniform percentage of reduction per pass. This exponentialcurve may be readily determined by an equation of the form where T1 isthe starting thickness, the number of passes, and F is the reducingfactor. The percentage of reduction will be where F is expressed as apercentage. According to my system the thickness of the work piece isdrastically reduced in at least two and preferably three successivepasses prior to the last reducing pass so that the thickness is lessafter any such pass than the thickness at that stage of the reductionwhich would result if a uniform percentage of reduction were obtained ineach stand. If this is done, the reduction in the final pass, where thewidth-thickness ratio of the material is the highest and hence where theproblem of guiding is most acute, may be as greator even greater thanthe reduction as computed by the above formula and yet the concavity,convexity or flatness. of the material from stand to stand follows norule but the work piece tracks through the mill and an entirelysatisfactory product is obtained.

My system should be distinguished from pack rolling systems which,generally speaking, are

concerned only with pieces of relatively short length. A difllcultypeculiar to the rolling of strip is that because of the great length ofthe work piece, any over-rolling, either of thecenter or of the edge, iscumulative to such a degree that the product may be spoiled; whereas inpack rolling the piece is so short that this difllculty is of muchsmaller consequence. My invention is matic, of a continuous mill inwhich my invention may be practiced;

Figure 2 is a. side elevation thereof; and Figure 3 is a curve sheetillustrating the reductions taken in each pass and the contour of thematerial from pass to pass.

The mill illustrated in Figures 1 and 2 comprises a series of reducingstands numbered successively from I to II and three sets of edging rollsI2 placed just in advance of each of stands 3, 4 and 5. Stands I and 2are 2-high, while stands 3 to II inclusive are all of the now wellknown4-high type. Stands I and,2 are driven through suitable connections I3and gearing I4 from a motor I5, while each of stands 3 to II' isprovided with an independent motor'drive indicated at IS. The workpiece, indicated at W, is supplied either direct from a universal millor from a heating furnace and is fed successively through the severalstands in the'mill. Tables I'I are provided between stands over whichthe piece travels. The final product may be cropped and/or cut intolengths by a rotary shear I8 and fed tocoilers I9 or run onto a coolingbed 20.

Stand No. II may be considered purely as a finishing pass intended toimpart the desired degree of flatness to the product. Stand I0 isproperly considered as the last reducing pass and for purposes of thepresent discussion stand No. I

may also be eliminated because the, work piece is has beenv plottedagainst the pass numberv and the'thickness at passes 2 to II inclusiveis shown. In this case the starting thickness was 1.981" and the finalthickness. was .098". These thicknesss are measured at the center of thestrip. The strip in this particular example was 37" wide. The thicknessat stand I0 maybe arbitrarily taken at .124, this leaving a sufficientlysmall amount of reduction to be effected in stand II to insure that thefinal product will have the desired characteristics.- Having thethickness at stand 2 and at stand III, the reduction which would obtainin each pass, were the same percentage of reduction to apply in eachcase, may be readily de termined by the above formula as follows:

Passes 2 to II] inclusive! total '9 in number. Hencen=9 and .1z4 F.0626, and

The reduction for the successive passes is therefore;l00-70.725=29.275%.Th curve 22 of Figure 3 is an exponential curve re resenting suchreduction frompasses 2 to l0 inclusive. It will be noted that the curve2| does not coincide with the curve 22 but departs materially.therefrom,

particularly at passes 4 to 6 inclusive. The actual thickness of thematerial after each pass, in this particular example of my invention, isas follows:

After pass No. 2 1.981 After pass No. 3 1.397 After pass No. 4 .868After pass No. 5 .617 After pass No. 6 .464 After pass No. I Q. .341After pass No. 8 .236 After pass No. 9 .184 After pass No. In .124

All thicknesses in the above tabulation are at the center of the strip.

In the rolling just described no attempt was made to initially determineor. maintain any fixed relationship between successive active passcontours. However, samples were taken after each pass from 2 to I0inclusive, and the convexity of the same was determined by subtractingthe average thickness of the edge from the thickness at the center. Theresults of the determination are plotted in Figure 3 by a broken' line23. The scale for the line 23 appears at the right-hand side of thefigure and gives the convexity in inches. It will be noted that there isno rational relationship whatever between the convexities in the severalpasses. The line 23 is only representative of this particular rollingbecause my tests show that in other rollings widely differingconvexities are obtained. There is no apparent relationship whateverbetween the lines plotted from different tests. Apparently all of thefactors enumerated above as bearing on roll pass contour have come intoplay but in such different ways and in such different amounts that thereis no fixed relationship whatever between the contours of the successivepasses. It may therefore be safely said that in my system of rollingconsideration of pass contour may be disregarded. I therefore am able togrind an ofthe rolls in the reducing passes to cylindrical form or, ifthis form is departed from in any pass, to determine the profile of theroll solely by considerations of ultimate flatness of the material andnot by considerations of rolling.

I have determined that even with the same set of rolls a mere change inthe speed of the mill will cause the active pass contours in successivepasses to vary markedly and in amounts difiering one from the other, sothat there is not even a rational relationship between the severalconvexities where the only change has been in the speed of the mill.

The reduction to thicknesses below the exponential curve should be intwo and preferably three successive passes, all of which are prior tothe last reducing pass. This merely requires a simple screw-downadjustment and a corresponding adjustment of the relative motor speed.It follows that the expensive and time-consuming adjustments of priorsystems are entirely eliminated and consequently a greater yield may beobtained from the mill. The amount of skilled supervision ferredembodiment of my invention, it will be understood that the same is notlimited thereto but maybe otherwise embodied and practiced within thescope of the following claims.

I claim:

1. In the method of rolling wide thin strip in a continuous mill havinga plurality of stands of reducing rolls, the steps consisting in passinga work piece'through the stands in a continuous manner, reducing thethickness of the work piece by single ply passes in each such stand, andin at least two successive passes, which passes are prior to the lastreducing pass, drastically reducing the thickness to less than thethickness at that stage of the reduction which would result if a uniformpercentage of reduction were taken in each stand to obtainthe same totalreduction.

2. In the method of rolling wide thin strip in a continuous mill havinga plurality of stands of reducing rolls, the steps consisting in passinga work piece through the stands in a continuous manner, reducing thethickness of the work piece by single ply passes in each such stand, andin at least three successive passes, which passes are prior to the lastreducing pass, drastically reducing the thickness to less than thethickness'at that stage of the reduction which would result if a uniformpercentage of reduction were taken in each stand toobtain the same totalreduction. 3. In the method of rolling wide thin strip in a continuousmill having a plurality of stands of reducing rolls, the stepsconsisting in heating a work piece to rolling temperature, passing itthrough the stands in a continuous manner, reducing the thickness of thework piece by single ply passes in each such stand, and in at least twosuccessive passes, which passes are prior to the last reducing pass,drastically reducing the thickness to less than the thickness at thatstage of the reduction which would result if a uniform percentage ofreduction were taken in each stand to obtain the same total reduction.

4. In the method of rolling wide thin strip in a continuous mill havinga plurality of stands of reducing rolls, the steps consisting in passinga work piece through the stands in a continuous manner, reducing thethickness of the work piece by single ply passes in each such stand, andin at least two successive passes, which passes are prior to the lastreducing pass, drastically reducing the thickness to less than thethickness at thatstage of the reduction which would result if a uniformpercentage of reduction were taken in each stand to obtain the sametotal reduction, and, subsequent to the reducing passes, subjecting thematerial in single thickness to a finishing pass.

5. In the method of rolling wide thin strip in a continuous mill havinga plurality of stands of reducing rolls, the steps consisting in passinga work piece through the stands in a continuous manner in single plypasses, and reducing the work piece in the intermediate passes so thatthe curve in which thickness is plotted against pass number lies, in anintermediate portion, sufllciently below an exponential curve of likecharacter connecting the first and last points of the first-mentionedcurve and representing uniform percentage of reduction per pass thatpredetermined relationships between successive active pass contours forthe purpose of guiding the work piece through the stands may bedispensed with.

CHARLES A. KRAL.

